This invention relates to polyphase modulation techniques in digital communication systems, and more particularly to method and apparatus for generating phase modulated carrier signals in data communication systems employing octal phase shift keyed (PSK) modulation.
In some phase modulated data systems, the phase of a carrier signal is periodically encoded according to digital words defined by groups of serial data bits. In such systems, a number of phasors or vector signals of the same frequency and amplitude and of different prescribed phases are required. A prior-art method of generating a 4-phase signal set is described at page 202, FIG. 10-1, of Data Transmission by W. R. Bennett and J. R. Davey, McGraw Hill Book Company, 1965. In this prior-art method, individual data bits of a pair thereof are applied to associated balanced mixers which are driven by quadrature phase carrier signals having phases of 0.degree. and 90.degree., for example. The phase of the signal from one modulator is 0.degree. and 180.degree. for an input data bit of 1 and 0, respectively. The phase of the signal from the other modulator is +90.degree. and -90.degree. when the associated data bit is 1 and 0, respectively. The modulator output signals are of the same amplitude. The outputs of the modulators are combined to provide a 4-phase signal set with phasors at .+-.45.degree. and .+-.135.degree.. A modified form of this technique is described there for generating an 8-phase signal set. It requires a pair of control signals, each having 4 different DC levels, for driving associated modulators to produce output signals which are combined to produce a resultant vector signal selectively having phases which are odd multiples of 22.5.degree.. It is difficult to consistently maintain the control signals at different prescribed DC levels so as to produce a set of 8 phasors with the same magnitudes and precise phases. The article "Modulating and Filtering For Three-b/Hz Operation At 6 GHz" by W. A. H. Wood, Ratheon Data Systems, at pgs. 60-61 of the October 1977 Communication News, describes a related technique of producing an 8-phase signal set. The technique in the article states that a carrier signal is split into orthogonal components and resistively divided to simultaneously form 8 equal amplitude vectors on separate output lines of the resistive divider. Vectors are then commutatively selected from associated output lines for transmission. Two such resistive-divider-commutator type generators are employed, with amplitude adjustment and phase adjustment in one of them, to generate a vector describing a 2.sup.n point vector phase. This technique is relatively complex for generating phasors of an 8-phase signal set.
An object of this invention is the provision of improved method and apparatus for producing a signal vector that generates phasors of an 8-phase signal set.